Several current production model helicopters use elastomeric bearings to support rotor blades from the rotor hub. Such bearings comprise stacks of alternating rubber and metal laminates in various geometric shapes, and relative motion between the blade and the hub is accommodated by incremental shear of each rubber laminate. One configuration for such a rotor is shown in U.S. Pat. No. 4,203,708 to Rybicki and is used on the Sikorsky S-76 model helicopter. An array of two elastomeric bearings is used in that configuration. Each blade is attached to its associated rotor hub arm by a root end blade shank portion called a spindle that extends through the center of both bearings and at its innermost end is attached to the inner race of the inboard bearing. A large nut threaded to the end of the spindle acts as a shoulder against the bearing race and carries the load from the spindle to the bearing array. Operational experience has shown that the dynamic components of the helicopter--most notably the rotor blades, rotor hub, and control linkages--are critical elements to the reliability of the aircraft. Engineering design efforts are continuously being expended to refine and improve these components to increase their strength, service life and reliability, and to decrease their weight, cost, drag, and maintenance requirements. These efforts include ideas and configurations directed towards redundancy, integrity monitoring, and adoption of fail-safe concepts. One example of a redundancy concept applied to a rotor head is illustrated by U.S. Pat. No. 3,761,199 to Ferris and Zincone. The patent covers a rotor hub having a central bore connected to the drive shaft and a series of radially extending spokes carrying a truss-shaped outer blade support. The truss design allows centrifugal, torsional, and thrust loads to be carried by the spoke members in the event of failure of the outer support.
An example of the integrity monitoring concept in this field is illustrated by U.S. Pat. No. 3,134,445 to Hotchkiss. The patent relates to the concept of applying a small amount of gas pressure to the hollow, sealed structural spar of the blade such that gas leakage due to a crack in the spar wall will be sensed and indication visibly displayed by a diaphragm as a warning to the operator of impending failure due to fracture of the spar. Several forms of this concept, including both ground and in-flight detectable systems, have been used to monitor the integrity of blade spars. These systems and related hardware are associated with the trademark BIM. One of the challenges to the helicopter dynamics designer in realizing the benefits of each of these systems is to design a component to incorporate both features. This involves the provision of redundancy such that a secondary system will take over full function upon failure of the primary system, together with an indication system that will provide the knowledge to the operator that the redundant or backup system has come into effect. For example, it is important to airplane flight safety that a second or backup wing beam take over structural support upon the failure of a primary beam, but it is just as important to alert the pilot that damage has occurred and should be corrected. Fail-safe systems are only the first steps in design and must be followed by a damage detection system.